MULTIPHOTON AC STARK-EFFECT IN A BICHROMATICALLY DRIVEN 2-LEVEL ATOM

We study the interaction of a two-level atom with two lasers of differ ent frequencies and amplitudes: a strong laser of Rabi frequency 2 Ohm (1) on resonance with the atomic transition, and a weaker laser detune d by subharmonics (2 Ohm(1)/n) of the Rabi frequency of the first. We find that under these conditions the second laser couples the dressed states created by the first in an n-photon process, resulting in ''dou bly dressed'' states and in a ''multiphoton ac Stark'' effect. We calc ulate the eigenstates of the doubly dressed atom and their energies, a nd illustrate the role of this multiphoton ac Stark effect in its fluo rescence, absorption, and Autler-Townes spectra. [S1050-2947(98)07607- 0].